Transmission of an MPEG transport stream is predicated on the use of a constant-delay transmission medium. A constant-delay transmission medium is a physical medium over which different MPEG data packets take the same amount of time to get from an MPEG transmitter/encoder to an MPEG decoder/receiver. The need for a constant-delay transmission medium is explained as follows.
The MPEG specifications call for an MPEG encoder to insert program clock references (PCRs) into MPEG transport streams containing MPEG-compressed audio and video data packets. PCRs are time-stamps used by an MPEG receiver to synchronize the presentation and recreation of audio and video information in the packets. The MPEG receiver uses information in the PCRs to generate local time-stamps, which are used in decoding the transport stream.
More specifically, when decoding a transport stream, the MPEG receiver compares the local time-stamps to PCRs inserted in the transport stream by the MPEG encoder. Differences between the local time-stamps and the PCRs constitute errors indicative of an instantaneous frequency difference between the clock that generated the PCRs and a local clock that generates the local time-stamps. The MPEG receiver uses these errors to synchronize the local clock with the clock that generated the PCRs. By synchronizing the clocks, the video and audio can be reproduced more accurately by the MPEG decoder/receiver.
When PCRs are transmitted from the MPEG encoder to the MPEG receiver at a rate that is not constant (this is referred to as “jitter” in the transmission), the foregoing clock synchronization process confuses error induced by non-constant data packet transmission with inherent error between the encoder (PCR) and receiver clocks. As a result, errors increase on average, causing unacceptably poor audio and video recovery by the MPEG decoder.